The present invention relates to a dynamic vibration absorber that reduces torsional vibration of a power transmission system caused in accordance with torque fluctuation of an automobile engine.
Conventionally, in fields such as automobiles, dynamic vibration absorbers have been proposed for reducing torsional vibration that occurs in power transmission systems. The dynamic vibration absorbers are typically mounted on a flywheel portion coupled to the crankshaft of the engine. Circular rolling element chambers are formed in the flywheel main body on which the dynamic vibration absorber is mounted. A spherical or roller type rolling element is accommodated in each of the rolling element chambers. In the dynamic vibration absorber, when the flywheel main body is rotated, the rolling elements that resonate with a predetermined order of the torsional vibration of the power transmission system roll in the rolling element chambers. Accordingly, a dynamic vibration absorbing operation is performed, and torsional vibration is absorbed.
In such a centrifugal pendulum dynamic vibration absorber, since each rolling element smoothly rolls along the rolling surface of the associated rolling element chamber when the rotational speed of the power transmission system is fast, the torsional vibration is absorbed. However, when the rotational speed of the power transmission system is slow such as when starting the engine or when the engine is stopped, the centrifugal force acting on the rolling elements is reduced, causing each rolling element to irregularly move in the associated rolling element chamber. Thus, the rolling elements collide against the inner wall of the rolling element chambers, which might generate noise and small vibration.
Japanese Laid-Open Patent Publication No. 7-280037 discloses a centrifugal pendulum vibration absorber that inhibits irregular movement of pendulums when the rotational speed of the power transmission system is reduced to prevent generation of noise and small vibration. The centrifugal pendulum vibration absorber disclosed in this document absorbs torsional vibration by resonance of the pendulums accommodated in the rolling element chambers. A stopper protrusion is formed at the center of the side surface of the rolling element, which is the centrifugal pendulum. A guide groove is formed in the side wall of each rolling element chamber.
The guide groove extends along an arc that is coaxial with the rolling surface of the rolling element chamber along which the rolling element rolls. With this structure, the stopper protrusion is loosely fitted in the guide groove, and the stopper protrusion abuts against the wall of the guide groove. Accordingly, the path of each rolling element is restricted to the arc extending along the rolling surface. The width and the length of the guide groove is set such that the stopper protrusion does not contact the wall of the guide groove during normal rolling of the rolling element, and the stopper protrusion abuts against the wall of the end portion of the guide groove only when the rolling element separates from the rolling surface or when the rolling element is rotated by a predetermined angle θ or more.
Also, an annular protrusion is formed at the peripheral edge of each rolling element. An annular groove is formed in the inner circumferential surface of each rolling element chamber. The annular protrusion of each rolling element is fitted in the annular groove of the associated rolling element chamber with a predetermined gap. The axial movement of each rolling element is restricted by fitting the annular protrusion in the annular groove. Each rolling element rolls along general part of the rolling surface except the annular groove.
A pair of the rolling element chambers provided with the guide grooves needs to be formed per each of the rolling elements. Also, highly accurate cutting is required for machining the guide grooves such that the rolling elements smoothly roll. This significantly increases the manufacturing costs. As described above, the structure of the centrifugal pendulum vibration absorber is very complicated. Furthermore, machining for manufacturing the centrifugal pendulum vibration absorber requires high dimension accuracy.
Moreover, although the stopper protrusion of each rolling element abuts against the wall of the associated guide groove to restrict the path of the rolling element as described above, collision noise occurs when the stopper protrusion abuts against the wall of the guide groove depending on the movement of the rolling element.